1. Field of the Invention
The present invention relates to a magnetoresistive device exhibiting a giant magnetoresistive effect, a thin film magnetic head, a head gimbal assembly and a magnetic disk unit comprising the magnetoresistive device, and more specifically the invention related to a magnetoresistive device in which a current flows in a laminate direction during use, and a thin film magnetic head, a head gimbal assembly and a magnetic disk unit comprising the magnetoresistive device.
2. Description of the Related Art
Conventionally, thin film magnetic heads comprising a MR device which exhibits a magnetoresistive effect (MR) are broadly used to reproduce information on magnetic recording media such as hard disks. In recent years, as the recording densities of the magnetic recording media increase, thin film magnetic heads using a giant magnetoresistive device (GMR device) which exhibits a giant magnetoresistive effect (GMR) are typically used. As such a GMR device, for example, a spin-valve (SV) type GMR device is cited.
The SV type GMR device comprises a SV film with a structure in which a magnetic layer (magnetization fixed layer) of which the magnetization direction is fixed in a predetermined direction and a magnetic layer (magnetization free layer) of which the magnetization direction changes depending upon an external signal magnetic field are laminated with a non-magnetic intermediate layer in between, and at the time of reproducing, a sense current flows through the SV type GMR device in an in-plane direction of a laminate. Such a GMR device is specifically called a CIP (Current in Plane)-GMR device. In this case, when a sense current flows depending upon a relative angle between the magnetization directions of two magnetic layers (that is, the magnetization fixed layer and the magnetization free layer) in the SV film, electrical resistance (that is, voltage) changes.
Recently, in order to respond to a further improvement in the recording density, the development of thin film magnetic heads comprising a CPP (Current Perpendicular to the Plane)-GMR device with a structure in which at the time of reproducing, a sense current flows in a laminate direction of the SV film has been pursued. Typically, such a CPP-GMR device comprises a SV film, a pair of magnetic domain control films which face each other so that the SV film is sandwiched therebetween with an insulating film in between in a direction corresponding to a reproducing track width direction, and a top electrode and a bottom electrode which are formed so that the SV film and the pair of magnetic domain control film are sandwiched therebetween in a laminate direction. The top electrode and the bottom electrode also serve as a top shield film and a bottom shield film, respectively. The CPP-GMR device with such a structure has an advantage that in the case where the dimension in a reproducing track width is reduced, the CPP-GMR device has a higher output than the CIP-GMR device. More specifically, in the CIP-GMR device, a sense current flows in a in-plane direction, so a magnetic sensitive portion where a sense current passes becomes minute with a decrease in the dimension in a reproducing track width direction, thereby the amount of voltage change is reduced. On the other hand, in CPP-GMR device, a sense current flows in a laminate direction, so a reduction in the dimension in a reproducing track width direction has a less influence on the amount of voltage change. Against such a background, expectations on the CPP-GMR device as a device capable of responding to a further improvement in the recording density expand.
Specifically, when a magnetization fixed layer has a three-layer synthetic structure including two ferromagnetic films (a first ferromagnetic film and a second ferromagnetic film) having magnetization directions fixed in antiparallel to each other and a non-magnetic film disposed between the ferromagnetic films, the net moment of the magnetization fixed layer is reduced, thereby even if an external magnetic field is applied, it is difficult to rotate the magnetization direction of the magnetization fixed layer, and antiferromagnetic coupling between two ferromagnetic films occurs, so the magnetization direction of the magnetization fixed layer is stabilized. In addition, when the net moment is reduced, a magnetostatic field in a magnetic head is reduced, and the symmetry of an output waveform is improved. A synthetic type CPP-GMR device comprising such a synthetic type magnetization fixed layer can deliver superior performance as a means for reproducing magnetically recorded information.
As a related art of the CPP-GMR device, for example, a CPP-GMR device comprising a magnetization free layer or a magnetization fixed layer with a multilayer structure in which ferromagnetic films and non-magnetic films are alternately laminated is disclosed (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-092826). The CPP-GMR device has such a multilayer structure so as to improve a spin-dependent scattering effect and obtain a large output. Moreover, a CIP-GMR device comprising a synthetic type magnetization fixed layer in which at least one of two magnetization fixed films constituting the synthetic type magnetization fixed layer has a multi-structure including an intermediate layer made of a nickel iron alloy (NiFe) (more specifically a structure of “cobalt (Co)/NiFe/cobalt (Co)”) is disclosed in Japanese Unexamined Patent Application Publication No. 2001-052317. In this case, when a ferromagnetic coupling magnetic field between a free layer and a magnetization fixed layer is reduced, a high magnetoresistive ratio can be obtained. Moreover, a synthetic type CPP-GMR device using a material with a negative bulk scattering coefficient β in at least one of a free layer and a magnetization fixed layer, and a material with a negative interface scattering coefficient γ in a non-magnetic intermediate layer sandwiched between the free layer and the magnetization fixed layer is disclosed in Japanese Unexamined Patent Application Publication No. 2003-298142. Such a structure is used to obtain a larger magnetoresistive ratio.
However, a conventional synthetic type CPP-GMR device has a structure in which the magnetizations of a first magnetization fixed layer and a second magnetization fixed layer are coupled in antiparallel to each other, so the net moment is reduced to stabilize the magnetization direction. On the other hand, a current flows through the SV film in a laminate direction, so there is a problem that a part of the amount of resistance change is lost due to the structural characteristics. In other words, the amount of resistance change (the amount of voltage change) by a GMR effect occurring between the first magnetization fixed film and the magnetization free layer and the amount of resistance change (the amount of voltage change) by a GMR effect occurring between the second magnetization fixed film having a magnetization direction opposite to the magnetization direction of the first magnetization fixed film and the magnetization free layer partially cancel each other out. In the description, an effect that the second magnetization fixed film increases the amount of resistance change is called “a positive resistance change”, and an effect that the second magnetization fixed film cancels the amounts of resistance change out is called “a negative resistance change”.